In chemical industry, process materials are often strong acids or alkali that tend to cause corrosion or dissolution of metals. It is possible to prevent corrosion caused by strong acids or alkali by using special metals, such as titanium, tantalum, zirconium, platinum, gold, rhodium, or alloys designed for different applications, in the manufacture of the frames of measuring sensors. Examples of such alloys are Hastelloy C and B, Monel, and Palladium titanium. However, strong chemicals dissolve even these materials, though slowly. The result is that ions of these metals end up in the solutions.
Especially in semiconductor industry, extraneous metal ions are not allowed, because they may ruin the semiconductor batch being manufactured.
Temperature measurement in the process is today made by a sensor with a frame made of metal material. The frame made of metal material is often formed of a pipe closed at the ends and a measuring element placed inside it. The measurement signal received from the measuring element is electric.
In semiconductor industry, parts installed into the process must be protected so that no metal is dissolved into the process. In these cases, the temperature sensors are coated with a suitable coating material, such as Teflon, Kynar, or some other similar coating. This type of coating protects against corrosion. However, acids penetrate even through Teflon, in which case the material of the frame begins to corrode in time.
The above-mentioned coating materials also have the disadvantage that they slow down heat transmission from the process to the sensor, since they are invariably also good heat insulators. The thickness of the coating materials cannot, however, be reduced much, because the coating then becomes vulnerable to mechanical damage and the metal surface of the sensor frame comes easily into contact with the process solution.
A coated sensor reacts slowly to temperature changes and, even at constant temperature, the reading remains as much as several degrees below the actual process temperature. This applies to high temperatures in particular.